Which method is NOT used to determine rate of a reaction?

Analyze the question: We are asked which method is not used to measure the rate of a reaction.

Concept: Reaction rate is how fast reactants are used up or products are formed. It is the change in concentration (or an equivalent measurable quantity) per unit time. So any method must track a property that changes as the reaction progresses over time.

Check each option:

• A. Change in amount of precipitate formed: Used. For example, in the reaction of sodium thiosulphate with HCl, sulphur precipitates and the time for a cross to disappear is measured.
• B. Change in equilibrium constant: Not used. The equilibrium constant (K) depends only on temperature and describes the position of equilibrium. It does not change as a reaction proceeds, and it says nothing about how fast equilibrium is reached.
• C. Change in intensity of colour: Used. Colorimetry tracks how the colour of a reactant or product changes with time (e.g., KMnO₄ fading).
• D. Change in pH values: Used. If H⁺ or OH⁻ is consumed or produced, the pH changes with time and can be monitored.
• E. Change in total gas pressure: Used. If gas is produced or consumed, the pressure change over time reflects the rate (e.g., a reaction in a closed vessel).

Answer: B. The equilibrium constant is constant at a given temperature, so it cannot be used to follow the progress of a reaction.

Tip: Rate = change in a measurable quantity ÷ time. Rate is a kinetics idea, while the equilibrium constant is a thermodynamics/equilibrium idea. If an option mentions "equilibrium constant" in a rate question, it is almost always the odd one out.